Composition for treating steels

ABSTRACT

A composition for treating molten steel comprising 3 - 40 wt.% of lithium fluoride, 15 - 50 wt.% of sodium fluoride and 30 - 70 wt.% of calcium fluoride and having a melting point between 600* and 1,100*C.

Unite States Patent Takashima Feb. 11, 1975 [54] COMPOSITION FOR TREATING STEELS 2,731,373 1/1956 Wasserman 148/26 2,805,178 9/1957 Gariott [75] Inventor: Masaru Takashlma, wakabayashl, 2,814,559 11/1957 Clark U Japan 3,031,346 4/1962 Wasserman 148/26 3,269,828 8/1966 Hale 75/53 [73] Assgnee' Tokyoto' Japan 3,272,667 9/1966 Siegle 148/26 [22] Filed: Jan. 16, 1973 3,446,614 /1969 Johnson 75/53 3,62l,188 Il/l97l Joseph 148/26 21 Appl. No.1 324,244

Primary Examiner-C. Lovell Foreign APPllcamm y Data Assistant Examiner-Peter D. Rosenberg Jan. 25, 1972 Japan 47-9220 Attorney, Agent, or FirmWenderoth, Lind & Ponack 52 US. Cl 75 58 75 53, 75 94, 1 148/06 57 ABSTRACT 'f CL Czlc 7/02 C2213 9/10 /34 A composition for treating molten steel comprising 3 [58] Field of Search 75/53, 58, 94; 148/26 of lithium fluoride 5 of Sodium fluoride and 30 wt.% of calcium fluoride and havl References Cited ing a melting point between 600 and 1,100C. UNITED STATES PATENTS 2.360.717 10/1944 P1161 4 /53 3 Drawmgs 1 .COMPOSITION FOR TREATING STEELS The present invention relates to a novel composition for treating molten steel which is effective to clean molten steel, particularly Al-killed molten steel prepared in various steel making furnaces, and is added to the molten steel during tapping and for ingot making steps. As compared with conventional compositions used for similar purposes, the present composition has remarkably enhanced cleaning effects and contributes to improve yield of steel ingot production.

Further the present composition is useful for production of various carbon steels, alloy steels, stainless steels and highly alloyed steels, and remarkably reduces both interior defects and surface defects of steel materials, thus greatly improving yield of steel products.

Regarding deoxidation of molten steel and removal of non-metallic inclusions, there are many prior publications, for example, U.S. Pat. No. 3,269,828. But effects and results obtained by the prior arts vary largely depending on their methods, types of additives or treating agents and their chemical analysis.

One of the objects of the present invention is to provide a novel composition for treating various kinds of steels.

The basic composition according to the present invention comprises:

3 40 wt.% of lithium fluoride 15 50 wt.% of sodium fluoride 30 70 wt.% of calcium fluoride and has a melting point between 600 and 1,lOC.

A modified composition according to the present invention comprises:

3 40 wt.% of lithium fluoride l 50 wt.% of sodium fluoride 3O 70 wt.% of calcium fluoride not more than 3 wt.% of hexafluorotitanic acid potassium; and/or not more than 3 wt.% of cerium fluoride and has a melting point between 600 and l,lO0C.

In a further modification, fluorides of rare earth elements or misch metal powders may be added in place of cerium fluoride.

In case of Al-killed molten steel, various kinds of non-metallic. inclusions are caused by entrappment of furnace slag during the teeming opertion, secondary oxidation of the molten steel in the air, the ladle reaction between molten steel and refractories and the melt down of refractories during the bottom-pouring ingot making operation. These non-metallic inclusions contain A type inclusions such as sulfides and silicates, B type granular inclusions such as alumina inclusions discontinuously grouping in the hot working direction, and C type inclusions such as granular oxides irregularly dispersed and not deformed by the hot working.

These inclusions cause crackings during the rolling or forging of the steel and deteriorate the mechanical properties of the steel, and particularly B type and C type inclusions cause fractures or fatigue raptures of the steel products in service.

In order to eliminate the causes of the above defects, it is necessary to float up and remove the non-metallic inclusions formed in the molten steel. [t has been found that this object can be achieved by introducing oxides which are able to combine with the formed oxides into the molten steel in the ladle or during the ingot-making process to produce low melting point coagulates by the reaction between the oxides and then by adding com positions which generate suitable gassing which intensifies collision action among the inclusions to accelerate the coagulation speed and enlargement of the coagulates, thus increasing the floating up of inclusions according to the flotation principle.

As a result of the above studies and researches, it has been confirmed that lithium oxide, which has larger coagulation action with respect to the oxides in the molten steel, is very useful; and for the source of the lithium oxide, a low melting point lithium fluoride has been selected. The lithium fluoride easily reacts with oxygen in the molten steel and forms lithium oxide which is an oxide having highly activated energy, and reacts with alumina and oxides formed in the molten steel and forms low melting point complexes and coagulates.

Sodium fluoride is also a low melting point fluoride and generates suitable gassing at the temperature of molten steel and is useful for a gas generating source for increasing the floating up-power of coagulated oxides.

Further, in order to adsorb and fix the floating oxides, it is necessary to provide a molten slag layer. For this purpose, calcium fluoride which is commonly used and forms a high fluidity slag is selected in the present invention.

The reaction which contributes to the removal of the non-metallic inclusions by bondage of oxides may be expressed as LiF O Li O F and some of the coagulated oxides are:

lei- 0 A1 0 Li O Al O SiO Li O manganese alumino-silicate.

The non-metallic inclusions given coagulation ability are adsorbed and fixed by the complex molten slag as the floating power is increased by the gas from sodium fluoride as well as by the vapour pressure of lithium fluoride and calcium fluoride.

Now, the reasons of limiting the compositions and the melting points as above will be explained below.

Less than 3 wt.% of lithium fluoride is not enough for producing Li O which forms neuclei of the oxide condensation, and on the other hand more than 40 wt.% of lithium fluoride produces excessive U 0 and destroys its precipitation balance.

Less than 15 wt.% of sodium fluoride is not enough as the gas generating source for floating of the coagulated oxides, but more than 50 wt.% of sodium fluoride causes sudden generation of an excessive amount of gas, and the smoking hinders the operation.

The amount of calcium fluoride should be proportionate to the formation of the molten slag layer necessary for adsorbing and fixing the floating oxides, and less than 30 wt.% of calcium fluoride is not enough for the above purpose and more than wt.% of calcium fluoride is excessive in view of the total slag amount effective for improving steel ingot surfaces. The proportions of the components are also adjusted in relation with the melting point of the compositions.

In the modified composition of the present invention, hexafluoro titanic acid potassium is added to the basic composition as hereinbefore stated. This additional component is effective to aid desulfurization of the molten steel and has a low melting point of about 780C and yet is lower in price than lithium fluoride, and thus useful for fine adjustment of the melting point of the composition. This effect can be attained with less than 3% thereof.

Deoxidation and desulfurization effects of cerium fluoride and its affinity for non-metallic inclusions such as A1 and SiO in molten steel are recognized in the art.

According to the present invention, cerium fluoride is added for the purpose of supplementary adjustment for cleaness of the molten steel in an amount not more than 3% in view of controlling precipitation of cerium oxide. The melting point of cerium fluoride is about 1,3 30C.

Further, the components of the composition according to the present invention are selected so as to have a melting point within a range of 600 l,lOOC. The melting points of simple fluorides are:

LiF 845C, NaF 990C and CaF l,4l8C and the eutectic temperatures of binary systems are:

LiF-NaF 652C, LiF-CaF 773C and NaF- CaF 818C and the ternary system of LiF--NaFCaF shows a lower eutectic temperature of 615C.

The mixture containing the above fluorides is divided into an appropriate particle size and mechanically mixed uniformly and added to molten steel in the form of powder, granules or briquette.

The proportions of the components of the composition according to the present invention are changed or adjusted in view of the amount of oxygen remaining in the molten steel which differs depending on the steel grades or types such as low-carbon steel grades, middle-carbon steel grades and high-carbon steel grades. The range of melting point of the composition is defined broad enough to cover the cases when lowmelting point CeF or K TiF is added.

The specific gravity of the composition of the present invention is desirably about 1.03 depending on the mixing proportion, and the viscosity is desirably about 0.15 poise at l,300C.

The cleaning effects attained by addition of the composition of the present invention include deoxidation of molten steel, removal of non-metallic inclusions and improvement of ingot surfaces as well as desulfurization and removal of hydrogen.

Among the above effects, deoxidatiom and removal of non-metallic inclusions by the composition of the present invention are as stated above.

The composition added to the molten steel forms molten slag having a low melting point and low viscosity, and rises together with the molten steel, covering the top surface of the molten steel in the mold during the teeming, and flows downs along the inside wall of the mold in a curtain-like form so that a smooth surface of steel ingot is obtained.

Further, LiF, NaF and CaF contained in the composition react with sulfur in the molten steel to form Li S,

Na s and CaS and thus desulfurization effect is ob-' tained. Further, remarkable desulfurization effect is obtained when CeF is also added.

Also, as the gas generating from the fluoride components of the compositionrises, H gas enriched during the steel making, tapping and ingot making steps is also taken up together with the gas, and thus the hydrogen gas content is reduced.

The composition of the present invention is added to molten steel in the following manners and amounts.

In case of bottom-pouring ingot making, the composition is added to the molten steel in an amount of to 1,000g per one metric ton of molten steel, and in case of top-pouring ingot-making, the composition is added in an amount of 300 to l,000g per one metric ton of molten metal. The addition of the composition may be carried out by putting part thereof within the mold or the pourer before the molten steel is poured, in an amount of 150 to 300g per one ton of molten steel, and adding the remaining part as required in the mold after the pouring is started, or the composition may be added to the stream of molten metal successively. In any way, the addition of the composition is completed before about half of the molten steel is poured.

The amount of the composition to be added before the pouring is determined by taking into consideration the method of ingot making and the size of ingots to be obtained.

The composition of the present invention can be added in a divided manner to tandishes for continuous steel casting and similar desirable results are obtained.

The composition of the present invention is applicable for treating various kinds of steels, and its chemical analysis is changed depending on the kind of steels. Preferred chemical analysis of the present composition are set forth under.

I. For treating AlSl 1040 carbon steel: LiF 4.5 5.5

wt.%, NaF 29.0 31.0 wt.% CaF 64.0 66.0 wt.%

2. For treating AlSI 302 stainless steel: LiF 34.5

35.5 wt.%, NaF 30.0 32.0 wt.% CaF 32.0 34.0 wt.%

3. For treating AlSI 4137 Cr-Mo steel: LiF 7.5 8.5

wt.%, NaF 43.0 45.0 wt.% CaF 47.0 49.0 wt.%

The above compositions may further comprise 1.0 wt.% of K TiF and/or CeF The examples of the present invention are shown in Tables 1 and 2.

Table 1 shows the chemical analysis of examples of the present composition; and Table 2 shows results obtained when the present composition is used in comparison with results obtained when the present composition is not used.

Test samples were prepared from 60 mm diameter steel bars rolled from a 6 ton steel ingot, and the estimation was made on the cleanness (.llS: Spot calculation method, number of viewed fields: 60 X 400) of the steel according to the JlSmicroscopic testing method for non-metallic inclusions.

The results clearly show that the non-metallic inclusions in the steel are remarkably reduced when the present composition is used.

Further, the surface of the steel ingot made with addition of the present composition was much smoother What is claimed is:

l. A composition for treating molten steel consisting essentially of 3-40 wt. of lithium fluoride, 15-50 wt. of sodium fluoride, 30-70 wt. of calcium fluoride and l-3 wt. of K TiF and having a melting point between 600 and l,lO0C.

2. A composition for treating molten steel consisting essentially of 3-40 wt. of lithium fluoride. 15-50 wt. of sodium fluoride, 30-70 wt. of calcium fluoride, l-3 wt. of K TiF and l-3 wt. of CeF and having a melting point between 600 and l,l00C.

3. A composition according to claim 1 which has a particle size under 40 mesh. 

1. A COMPOSITION FOR TREATING MOLTEN STEEL CONSISTING ESSENTIALLY OF 3-40 WT. % OF LITHIUM FLUORIDE, 15-50 WT. % OF SODIUM FLUORIDE, 30-70 WT. % OF CALCIUM FLUORIDE AND 1-3 WT. % OF K2TIF6, AND HAVING A MELTING POINT BETWEEN 600* AND 1,100*C.
 2. A composition for treating molten steel consisting essentially of 3-40 wt. % of lithium fluoride, 15-50 wt. % of sodium fluoride, 30-70 wt. % of calcium fluoride, 1-3 wt. % of K2TiF6 and 1-3 wt. % of CeF3, and having a melting point between 600* and 1,100*C.
 3. A composition according to claim 1 which has a particle size under 40 mesh. 